The present invention relates to fixtures for holding workpieces during machining, inspection or other operations performed thereon. In particular, aspects of the present invention relate to a fixture for holding large workpieces having surfaces of high curvature or other complex shapes.
Holding fixtures for three dimensional workpieces are available from Modig Machine AB of Virserum, Sweden and CNA Manufacturing Systems, Inc. of Woodinville, Wash. These fixtures hold workpieces in a fixed position through an array of posts. Each of the posts are equipped with a flexible vacuum cup that when seated upon a portion of the surface of the workpiece forms a vacuum. When a vacuum is pulled in each of the posts, the workpiece is held securely. The posts are arranged in a grid pattern, oriented vertically, such that when the vacuum cups are in a home or resting position each of the vacuum cups lay in a common plane. In order to hold a three dimensional part, each of the posts are adjusted vertically so as to conform the posts taken as a whole to the outer surface of the workpiece to be held. In the Modig System, a large setting machine is provided to adjust the posts. In particular, a gantry setting machine includes two guide rails positioned along opposite sides of the grid of posts and a bridge spanning between the guide rails carries a plurality of setting fixtures used to set the vertical height of each of the posts. As indicated above, the posts are arranged in a grid pattern of equally spaced rows and columns. The setting device, disposed on the bridge, conforms to the row spacing of the posts in the grid. As the bridge moves along the guide rails, the height of each of the setting devices on the bridge is adjusted such that when the corresponding post, located below each setting device, is actuated the post extends upwardly until the desired height of the vacuum cup is obtained. The bridge is moved along the grid on a column-by-column basis setting each column of posts of the grid in this manner. The grid pattern of the posts and the setting bridge may limit the extent of curvature that the workpieces can have.
The vacuum cup of each post is coupled to an actuator rod of the post with a flexible coupling. The vacuum cup is designed to be as flexible as possible to clamp workpieces under vacuum with a steep contact angle to the reference ball of the coupling or the axis of the actuator. The design requires a very flexible or loose coupling between the vacuum cup and the actuator rod such that when a vacuum is drawn, the vacuum cup can orient itself and properly seat upon the workpiece. However, for workpieces having high curvature, this may cause the loose coupling to drop under gravity, rendering it difficult to orient the vacuum cup normal to the surface of the workpiece in order to be properly seated.
It is the object of the present invention to provide an improved holding fixture or assembly that can hold workpieces having surfaces of high curvature or complex shapes. Aspects of the present invention individually address the foregoing problems.